The present invention relates to circuit-switched networks, but more specifically to a method and an apparatus for determining paths for data packets or synchronous optical network/synchronous digital hierarchy (“SONET/SDH”) connections in a network during link failures.
While the disclosure sets forth an embodiment of the invention with respect to transmission of data packets, it also applies to SONET/SDH circuits as well.
Data networks, such as asynchronous transfer mode (“ATM”), frame relay (“FR”), public-switched telephone networks (“PSTN”), optical cross-connect (“OXC”), packet layer (“PL”), and IP multiprotocol label switching (“IP/MPLS”) include nodes connected by links. There could be multiple links between a pair of nodes in a network. A typical node controller implements open shortest path first (“OSPF”), multi protocol label switching (“MPLS”), private network node interface (“PNNI”) or other routing and signaling protocols. Data packets transmitted in such networks travel between these nodes over the links. To determine a path through the network, a node controller uses an assigned weight associated with prospective links to determine a best or efficient path that will complete the circuit from a source node to a destination node. Weights may also be used to characterize a number of Quality of Service metrics associated with the link, which the controller may also assess in determining the desirability of a prospective path.
Based on these weighting factors, the node controller determines the appropriate service path, and also sends setup messages to provision all nodes in the path according to the type of information to be conveyed. In addition to determining a service path, the controller also determines a restoration path in the event of a subsequent link failure in the service path. In prior systems, both service and restoration paths were generally determined by first eliminating from consideration those links that do not meet the Quality of Service requirements, and then using the assigned weights of the remaining links in a Dijkstra-like algorithm to calculate a minimum weight path to a destination. The elimination of links that do not meet Quality of Service requirements can also be embedded inside the Dijkstra algorithm.
The Dijkstra algorithm identifies the path with the lowest weight based on link weighting factors when conveying information from an originating node to a destination node in the network. The link weighting factor is generally a static value assigned to each link and may represent any factor to determine an optimal path, including such factors as the length of the link, cost of the link, and bandwidth capacity of the link. One example of a commonly used link weighting factor is the inverse of the bandwidth. Because the Dijkstra algorithm chooses a path with the lowest weight, it will identify the path among many with the greatest bandwidth. The algorithm includes comparing link weight values of the links directly connecting the source node to adjacent nodes in the network. The link with the lowest weighting value is selected and stored as the optimal path between a source node to a corresponding adjacent node. The algorithm then repeats this process, comparing the weights among alternative paths for the remaining links at link junction with the sum of the link value chosen in the first step, and the links directly connecting the node chosen in the first step with any other nodes. The path with the lowest sum total weighting value is chosen as the optimal path for conveying the data packets to the destination node. The algorithm and solution continue to expand until the optimal path for the destination node is found. The node controller then sends setup messages to provision the nodes identified in the optimal path and sends the data packets over that path. Service paths are typically provisioned one at a time, whenever the requests arrive at the nodes.
In the event of a network failure, many circuits may be affected. The node controllers attempt to restore the circuits as quickly as possible to keep the service outage to a minimum. These circuits may originate in different nodes and may be restored simultaneously in a distributed fashion by many node controllers in the network independently of each other. The distributed restoration may become stale, e.g., out-of-date, relative to available capacity of other links after a network failure because a minimum amount of time is necessary for routing messages through the network to update the state of the network. Due to out-of-date information, the path chosen for restoring a circuit may not work out. As the node controller attempts to set up the circuit along the path, it may find that a link in the path no longer has available capacity because the capacity was assigned to other circuits that just restored. The setup message for the circuit then cranks back to the node controller. The node controller will eliminate this link from consideration and calculate a new path. There is a need to minimize the number of crankbacks as these prolong the time it takes to restore the circuit. The smaller the amount of available capacity in a link, the higher the likelihood of a crankback. Thus, there is a need to determine different paths for the service path and the restoration path as this may lead to the maximum available capacity for restoration. During restoration, the node controller may need to identify paths for many failed circuits in a very short time. The path need not be the shortest possible path because it only needs to be used until the network failure is repaired and the restored circuits routed back to their original service paths. When determining the original service path, the node controllers must prioritize finding the shortest possible path; however, there is no constraint on this time period. An alternative procedure used in the art is for the controller to pre-calculate a prospective restoration path for a circuit, store it, and use it first when the circuit fails. If the setup message cranks back on this path, then the Dijkstra algorithm is used to obtain a new path.
In view of the foregoing, the present invention addresses the need for different path calculations for service provisioning and restoration by assigning each link at least two different weights, one being used to calculate the service path and the other being used to calculate the restoration path.